Show that cos²π/10+cos²4π/10+cos² 6π/10+cos²9π/10=2. I am a bit confused if I make Cos²4π/10=cos²(π-6π/10) & cos²9π/10=cos²(π-π/10), it will turn negative as cos(180°-theta)=-costheta in the second quadrant. How do I go about proving the question?
Please see below.
We know that,
getting back to the question,
P.S. you were going right, just note that even if its negative, the final answer turns out to be positive as the